The sensors are for example used in systems for observing the Earth by satellite. They comprise a plurality of parallel rows of photosensitive pixels; the sequencing of circuits for controlling the various rows (control of exposure time then read out of the photogenerated charges) is synchronized with the relative movement of the scene and sensor, so that all the rows of the sensor see a single line of the observed scene. The generated signals are then added point-by-point for each point of the observed line.
The theoretical signal-to-noise ratio is improved proportionally to the square root of the number N of rows in the sensor. This number may range from a few rows to about a hundred rows depending on the application (industrial quality control, terrestrial observation, panoramic dental radiography or mammography).
In charge transfer sensors (CCD sensors), point-by-point addition of the signals is achieved naturally without read noise by emptying into a row of pixels the charges generated and accumulated in the preceding row of pixels, in sync with the relative movement of the scene and sensor. The last row of pixels, having accumulated N times the charges generated by the observed scene line, may then be transferred to an output register and converted, in a read phase, into an electrical voltage or current.
Such charge transfer sensors are either made in conventional technologies with adjacent transfer gates produced from at least two polysilicon levels, the second level partially covering the first, or in technologies employing a single polysilicon gate level, these single-level technologies being more compatible with current technologies for manufacturing CMOS logic integrated circuits.
However, charge sensors using active CMOS technology pixels have advantages and an example thereof has been described in patent application WO 2008034794. Charges are not transferred row-to-row since the active pixels do not operate in a charge transfer mode but deliver a voltage to a column conductor. In order to add the signals corresponding to a given image line seen by the various pixel rows, an analog/digital conversion is used to deliver a digital representation of the output of each pixel, and N digital values issued from N pixels that have seen the image point in succession during the movement are added. However, the principle described in this application does not allow veritable correlated double sampling read out.
In addition, one problem encountered with time-delay and integration sensors is degradation of the modulation transfer function due to the fact that the relative movement of the scene in front of the sensor is continuous whereas the pixel information is processed discretely. Thus, an image of black and white bars of the pitch of the pixels, which would deliver, as output from the sensor, a peak-to-peak signal amplitude of unitary value if the image were static (omitting the purely geometric modulation transfer function) delivers only a peak-to-peak amplitude of 0.64 if the image is moving. This value of 0.64 is the movement component of the modulation transfer function (there are other components related to other factors which may further degrade the overall modulation transfer function).
Of course, the size of the pixels could be decreased in order to compensate for the poor modulation transfer function by increasing theoretical resolution. By dividing the pitch of the pixels by two, resolution is multiplied by two. However, in this case the number of analog/digital converters must also be multiplied.
Another constraint on image capture with CMOS technology pixels is the need to expose all the rows of pixels during the same time window of adjustable duration if possible (global shutter operating mode) and not during successive windows of the same duration but shifted in time from one row to the next (rolling shutter operating mode).
Lastly, it will be recalled that image capture with CMOS technology pixels comprising four or five transistors is subject to kTC read noise that it is necessary to attempt to reduce by carrying out a correlated double sampling read out; this means that it is necessary first to attempt to read a reset level of a charge storage node before transferring active charges to this node. In known CMOS technology sensors, the global shutter operating mode is incompatible with veritable correlated double sampling.